In the drilling of oil, gas and geothermal wells, strings of drill pipe and other well pipe must be made up as the string is run into the well, and disassembled into individual lengths of pipe (referred to as joints) as the string is withdrawn from the well. The successive joints of well pipe are connected together by screw thread connectors. When the well depth is shallow, making and breaking the threaded connections can be done with relative ease, as by using pipe tongs manipulated by hand. In the case of deep wells, many more joints of pipe are required and it becomes necessary to speed up the making and breaking of the connections between joints and to minimize manual labor. In all cases, it is desirable, and in the case of a deep well essential, to minimize the time required to accomplish a "trip" of the pipe out of and into the well. Thus, for example, to replace a worn drill bit the entire drill string must be pulled from the drill hole, with each of the many threaded connections being broken as the pipe is pulled, the drill bit then being replaced, and the entire string then being reassembled again as the new bit and string are run into the hole. Prior art workers have accordingly developed power operated tongs of various kinds and power operated spinners, the power tongs being capable of initially breaking the threaded connections and of final tightening them, and the power operated spinners serving to rotate the pipe rapidly during final unthreading or initial makeup of the connection. The tongs and spinners have been installed on the drilling rig in various fashions.
Apparatus according to the invention is applicable to all drilling procedures which require a well pipe structure in which joints of pipe are connected by screw thread connectors, including but not limited to rotary table drilling and rotary drilling with the use of power swivels. During rotary table drilling a length of rod of square transverse cross section, called a Kelly, is connected to the drill string and is engaged by the drive bushing of a rotary table at the rig floor, the Kelly descending through the drive bushing as drilling procedes. Alternatively, the Kelly is replaced by a shorter device, such as a Kelly sub. In either case, each time drilling has proceeded far enough to require that a new joint of pipe be added to the string, it is necessary to support the string, as with slips, detach the Kelly or sub, attach the Kelly or sub to the upper end of the new joint of pipe, then attach the other end of the new joint to the string. To facilitate such a series of operations, it has become standard practice to provide in the rig floor, in a location adjacent to the string but spaced laterally therefrom, a receptacle to accommodate the new joint of pipe, this receptacle being called a mousehole. It is therefor desirable that the power tongs, as well as the spinner, be of such nature as to be positionable selectively in alignment with the axis of the drill string, i.e., at well center, and in alignment with the mousehole, as well as in a stowed location spaced from both the well center and the mousehole. Such mobile apparatus have been proposed, as seen for example in U.S. Pat. No. 4,348,920 to Boyadjieff. However, though such apparatus have the advantage of satisfying the need for the multi-position capability, the manner in which mobility has been achieved tends toward instability and undue complication and there has been a continuing need for improvement.